Objects with low SB (surface brightness) are difficult to see.
Some examples are Barnard's Galaxy NGC 6822, the Helix planetary nebula NGC 7293 and the Running Chicken nebula IC 2948.
http://antwrp.gsfc.nasa.gov/apod/ap020123.html
http://antwrp.gsfc.nasa.gov/apod/ap070803.html
http://apod.nasa.gov/apod/ap080418.html
A UHC or OIII nebula filter greatly enhance the last two objects.
I find that sweeping the telescope or eye slowly back and forth helps with all of these objects. The eye detects movement very well. Also I look at all the faint stars around an object one by one, and use averted vision to look in the 2 o'clock position next to an object.
Nights with a breeze and no dew are better for seeing things with low SB. I saw Comet Boattini's tail on a recent windy night by sweeping back and forth at 90 degrees across the tail.
Please add your hints for seeing low SB objects.